In electrostatic printing and copying systems, an electrostatic writing head is commonly used to deposit a latent image of electrical charges onto a recording medium such as a paper web. The paper web carrying the latent image is then directed to a liquid toning applicator which deposits charged toner particles onto the paper web, thereby developing the latent image. The paper web may then be passed to a separate structure containing an air knife which uses the shearing force produced by a stream of high pressure air expelled over a knife edge surface to remove excess toner remaining on the surface of the recording medium. Air knives are well known in the art for removing liquids from moving webs in industrial applications such as oil removal from metal webs in aluminum rolling mills. Additionally, U.S. Pat. No. 4,870,462 to G. F. Day teaches the use of air knives for removing liquid toner from a recording medium.
Although air knives are useful for removing excess toner, air knife structures, which are separate from the applicator structure, also increase the size and cost of electrostatic printing and copying systems. Because the air knife is separate from the applicator, costs associated with manufacturing an additional mechanical structure are incurred. Additionally, separate toner drain lines or buckets to hold the excess toner removed from the recording medium by the air knife are required. The implementation and use of such drain lines and buckets increase the cost of the toner system. Furthermore, the drain lines and buckets also require occasional cleaning.
Air supply means must also be provided for the air knife. A separate air supply means, such as a blower or an air pump, may be used, or the air knife may be coupled to a pump or blower present in the toner applicator using connecting hoses or plumbing. In either case, the use of an air knife separate from the toner applicator adds additional components to the toner system. The additional components lead to an increase in the size and cost of the applicator system.
The problems associated with separately arranged air knives are compounded in single-applicator systems in which a single applicator is used to apply multiple colored toners to a recording medium. As mentioned above, the air knife must be provided with a separate drain line or bucket to receive the toner removed by the air knife from the recording medium. However, in order to avoid mixing of the various colored toners, the drain line or bucket of the air knife must be cleaned after each application of differently colored toner to the recording medium. This cleaning process is both difficult and time consuming.
The use of a separately situated air knife is also inefficient in a positive air pressure toner applicator system such as is described in co-pending U.S. Pat. application Ser. No. 07/998,458. In such a system, positive air pressure is used to confine the toner fluids to the surface of the applicator. Air is expelled at the edges of the applicator surface, at a pressure greater than the pressure of the fluids confined within the applicator. The high pressure air prevents fluid leakage from the edges of the toner applicator, thereby eliminating the need for a funnel or bucket to catch spilled toner. In so doing, a difficult cleaning problem is eliminated, and a single applicator may be used for multiple color toning. However, because the air knife is separate from the applicator, a pressurized air channel and drain line must be provided at the web exiting edge of the applicator to prevent toner leakage from that edge in order to avoid the need for a funnel or a bucket. Additionally, an air channel and a toner drain line or bucket must be provided in the air knife as well. Thus, using a separately situated air knife requires forming one air channel and drain line for the applicator, and a second air channel and drain line or bucket for the air knife.
Therefore it is an object of this invention is to provide a low-cost air knife for removing excess toner from a recording medium which does not increase the size or manufacturing requirements of the toner system.
A further object of this invention is to provide such an air knife which reduces the number of air channels and drain lines or buckets required in positive pressure toner applicator systems.